U.S. patent application number 10/735871 was filed with the patent office on 2004-07-01 for diffuser for a centrifugal compressor.
This patent application is currently assigned to NUOVO PIGNONE HOLDING S.p.A.. Invention is credited to Baldassarre, Leonardo, Camatti, Massimo.
Application Number | 20040126230 10/735871 |
Document ID | / |
Family ID | 31726554 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040126230 |
Kind Code |
A1 |
Baldassarre, Leonardo ; et
al. |
July 1, 2004 |
Diffuser for a centrifugal compressor
Abstract
An improved diffuser (10) for a centrifugal compressor which
comprises blading with blades (12).
Inventors: |
Baldassarre, Leonardo;
(Florence, IT) ; Camatti, Massimo; (Serravalle
Pistoiese, IT) |
Correspondence
Address: |
NIXON & VANDERHYE P.C./G.E.
1100 N. GLEBE RD.
SUITE 800
ARLINGTON
VA
22201
US
|
Assignee: |
NUOVO PIGNONE HOLDING
S.p.A.
Florence
IT
|
Family ID: |
31726554 |
Appl. No.: |
10/735871 |
Filed: |
December 16, 2003 |
Current U.S.
Class: |
415/208.2 |
Current CPC
Class: |
F05D 2250/52 20130101;
F04D 29/444 20130101 |
Class at
Publication: |
415/208.2 |
International
Class: |
F03B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 17, 2002 |
IT |
MI2002A 002661 |
Claims
1. Improved diffuser (10) for a centrifugal compressor,
characterised in that it comprises blading with blades (12).
2. Improved diffuser (10) according to claim 1, characterised in
that the said blading has a strength s of the said blades (12)
which is between 0.5 and 1, including extreme values, the said
strength s being provided by the ratio between the pitch p of the
said blading and the chord c of the said blades (12), the said
pitch p being provided by the ratio 2 Dp_in Z ,wherein Z is the
number of the said blades (12) and Dp in is the diameter of an
intake edge of the said blading.
3. Improved diffuser (10) according to claim 1 or claim 2,
characterised in that a deflection .beta. of the said blading, i.e.
the angle of displacement of a tangent line at the outlet of the
blade (12) relative to a tangent line at the intake of the blade
(12), is between an angle of 0.degree. and an angle of 10.degree.,
including extreme values.
4. Improved diffuser (10) according to claim 1 or claim 2 or claim
3, characterised in that the ratio between a diameter of an intake
edge Dp in of the said blading and an outer diameter of an impeller
D2 of the said centrifugal compressor, is between 1.04 and 1.14,
including extreme values.
5. Improved diffuser (10) according to claim 1 or claim 2 or claim
3 or claim 4, characterised in that the ratio between a diameter of
an outlet edge Dp out of the said blading and an outer diameter of
an impeller D2 of the said centrifugal compressor, is between 1.25
and 1.35, including extreme values.
6. Improved diffuser (10) according to claim 1 or claim 2 or claim
3 or claim 4 or claim 5, characterised in that it is used in
centrifugal compressor stages with a coefficient of flow of 0.03 or
less.
7. Improved diffuser (10) according to claim 1, characterised in
that a design of the said blades (12) is optimised by means of the
so-called CFD i.e. Computational Fluid Dynamic method (in other
words a method for fluid-dynamics calculation).
8. Improved diffuser (10) according to claim 1, characterised in
that a design of the said blades (12) is optimised by means of
experimental methodology.
9. Improved diffuser (10) according to claim 1, characterised in
that it is used for delivery of a centrifugal compressor for
re-injection.
10. Improved diffuser (10) for a centrifugal compressor,
substantially as described and illustrated and for the purposes
specified.
Description
[0001] The present invention relates to an improved diffuser for a
centrifugal compressor.
[0002] As is known, a centrifugal compressor is a machine which
returns a compressible fluid at a pressure which is greater than
that at which it received the fluid, by imparting to the fluid the
energy necessary for the change of pressure, by means of use of one
or a plurality of rotors or impellers.
[0003] Each rotor comprises a certain number of blades, which are
disposed radially such as to form a certain number of passages
which converge towards the centre of the rotor.
[0004] In high-pressure centrifugal compressors, the impellers
rotate in stators which comprise an inner case, diffusers and
diaphragms.
[0005] From the point of view of the performance of the centrifugal
compressor, there are two main aspects to be taken into
consideration, i.e. the polytropic output (in particular from the
design point of view) and the operative field.
[0006] A phenomenon which is particularly important, especially in
the field of high-pressure machines, is that of rotary stall of the
diffuser.
[0007] As is known, when the flow rate produced by the machine is
reduced, the gas tends to enter the diffuser with angles which are
increasingly small (relative to the tangential direction). When a
minimum value of this angle is reached, the diffuser reaches the
condition of rotary stall.
[0008] This condition is characterised by the occurrence of
pressure pulses at low frequency (the ratio between the pulse
frequency and that of rotation is normally between 0.1 and 0.2).
The intensity of the pulses is directly proportional to the density
of the gas, and thus to the pressure of the gas inside the
diffuser.
[0009] It can then clearly be understood that on high-pressure
machines these pulses tend to become particularly strong, to the
extent in fact that these oscillating forces lead to equally
violent vibrations of the shaft, thus preventing use of the machine
itself. The presence of this phenomenon thus gives rise to
limitation of the use of the machine solely to a specific field of
operative conditions (with low flow rates).
[0010] The solution used in order to mitigate this phenomenon, i.e.
in other words to displace the rotary stall outside the contractual
operative field, usually consists of reducing the opening for
passage of the gas into the diffuser.
[0011] For the same flow rate produced by the machine, this
therefore provides the effect of increasing the angle of the gas in
the diffuser, and thus of averting the critical conditions of
occurrence of the phenomenon.
[0012] However, the reduction of the opening for passage into the
diffuser has important consequences on the efficiency of the stage
concerned, and of the machine.
[0013] In fact, with the restrictions of the opening which are
normally required, and are necessary in order to solve the problem,
and can for example be 30% of the opening of the impeller, there is
penalisation which can be as much as 5% of the output of the
stage.
[0014] The object of the present invention is thus to eliminate the
disadvantages previously described, and in particular to provide an
improved diffuser for a centrifugal compressor, which makes it
possible to displace the phenomenon of rotary stall outside the
contractual operative field, whilst however maintaining a high
level of performance of the stage, which is even better than that
which can be obtained with a diffuser according to the known art,
with an opening with a reduced passage.
[0015] Another object of the present invention is to provide an
improved diffuser for a centrifugal compressor, which comprises an
increase in the operative field of the machine.
[0016] Another object of the present invention is to provide an
improved diffuser for a centrifugal compressor, which is
particularly reliable, functional, and has relatively low
costs.
[0017] These objects and others according to the invention are
achieved by providing an improved diffuser for a centrifugal
compressor, as described in claim 1.
[0018] Further characteristics of an improved diffuser for a
centrifugal compressor are indicated in the successive claims.
[0019] The characteristics and advantages of an improved diffuser
for a centrifugal compressor according to the present invention
will become more apparent and evident from the following
description, provided by way of non-limiting example, with
reference to the attached schematic drawings, in which:
[0020] FIG. 1 is a diagram of a portion of an improved diffuser for
a centrifugal compressor according to the present invention,
showing blading wherein the median lines of the blades are
drawn;
[0021] FIG. 2 shows an elevated lateral view of a portion of an
impeller and diffuser assembly according to FIG. 1; and
[0022] FIG. 3 is an elevated front view of a blade of the blading
in FIG. 1.
[0023] With initial reference to FIGS. 1 and 2, there is shown an
improved diffuser, indicated as 10 as a whole, for a centrifugal
compressor.
[0024] In the example illustrated, according to the present
invention, the diffuser 10 comprises substantially blading with
blades 12.
[0025] For the purposes of specifying an arrangement of the blades
12, the following variables, which are indicated in FIGS. 1 and 2,
are introduced:
[0026] D2, i.e. the outer diameter of an impeller of the
centrifugal compressor;
[0027] Dp in, i.e. the diameter of an intake edge of the
blading;
[0028] Dp out, i.e. the diameter of an outlet edge of the
blading;
[0029] .beta., i.e. the deflection of the blading, in other words
the angle of displacement of a tangent line at the outlet of the
blade 12, relative to a tangent line at the intake of the blade 12
itself;
[0030] p, i.e. the blading pitch of the diffuser, in other words 1
Dp_in Z ,
[0031] wherein Z is the number of the blades 12; and
[0032] c, i.e. length of the blades 12, which is also known as the
chord.
[0033] Other important variables are:
[0034] b2, i.e. outlet width of the impeller;
[0035] b3, i.e. width of the diffuser;
[0036] s, i.e. strength of the blade 12, provided by the ratio
between p and c, in other words between the diffuser blading pitch
and the chord of the blade 12.
[0037] The aforementioned variables are now indicated with
numerical intervals for satisfactory operation, with particular
reference to the positioning of the intake and outlet edge of the
blades 12, the strength s of the blade 12, and the deflection
.beta. of the blading.
[0038] The positioning of the blades 12 is provided by one or both
of the following ratios with reference to the outer diameter of the
impeller D2:
[0039] (Dp in)/D2 between 1.04 and 1.14 with extreme values
included;
[0040] (Dp out)/D2 between 1.25 and 1.35 with extreme values
included.
[0041] The optimal deflection .beta. of the blading is between an
angle of 0.degree. and an angle of 10.degree., including extreme
values.
[0042] The strength s of the blade 12 has low values and an optimal
configuration has been determined for values of between 0.5 and 1,
including extreme values.
[0043] The preferred field of use is in centrifugal compressor
stages with a coefficient of flow of 0.03 or less.
[0044] Advantageously, the design of the blades 12 can be optimised
both by means of the so-called CFD, i.e. Computational Fluid
Dynamic method (in other words a method for fluid-dynamics
calculation), and by means of experimental methodology.
[0045] By means of the improved diffuser according to the
invention, it is not necessary to implement any additional
reduction of area of the diffuser.
[0046] Experimental tests show that it is possible to obtain
substantial increases of performance (of up to five percentile
points) compared with the known configuration of free vortex
diffusers with a passage opening which is not reduced.
[0047] It is also found that there are substantial increases in the
operative field of the centrifugal compressor; the rotary stall
limit obtained coincides substantially with that of a free-vortex
diffuser with a reduced opening (30% of the discharge opening of
the impeller).
[0048] An application which is particularly suitable for the
improved diffuser for a centrifugal compressor, according to the
present invention, is that in a delivery diffuser of a centrifugal
compressor for re-injection.
[0049] The description provided makes apparent the characteristics
of the improved diffuser according to the present invention for a
centrifugal compressor, and also makes apparent its advantages.
[0050] The following concluding points and comments are now made,
such as to define the said advantages more clearly and
accurately.
[0051] Firstly, it is found that the improved diffuser 10 makes it
possible to displace the phenomenon of rotary stall outside the
contractual operative field, whilst however maintaining a high
level of performance of the stage, which in fact is better than
that which can be obtained by means of a diffuser according to the
known art, with a passage opening which is not reduced.
[0052] In addition, by means of the diffuser according to the
invention, it is found that there is an increase in the operative
field of the centrifugal compressor.
[0053] Furthermore, it is found that the improved diffuser of the
invention, for a centrifugal compressor, is particularly reliable
and has costs which are relatively low compared with the advantages
obtained.
[0054] Finally, it is apparent that many modifications and
variations, all of which come within the scope of the invention,
can be made to the improved diffuser for a centrifugal compressor
thus designed; in addition all the details can be replaced by
technically equivalent elements. In practice, any materials, forms
and dimensions can be used, according to technical
requirements.
[0055] The scope of the invention is thus delimited by the attached
claims.
* * * * *